rebase: test abort immediately after init
rebase: test abort immediately after init

Instead of `open`ing a rebase and `abort`ing that, test that we can
`abort` a rebase that has just begun with `init`.

Corillian/libgit2,Corillian/libgit2,stewid/libgit2,Corillian/libgit2,stewid/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2